The use of play building elements which are interconnected to form play structures is well known in the art. For example, cylindrical, circular and square building elements have been sold in the United States under the name "Crystal Climbers". The square building elements included an inwardly directed rectangular slot in the center of each side which was somewhat wider than the thickness of the various elements so that a fair amount of play was provided when respective slots were mutually received in one another. Slots were also provided radially in the circular elements and axially in the cylindrical elements. These elements were made of a clear plastic material and were not of sufficient size, strength, or of a sufficiently tight connection to make structures which would support the weight of a child.
Various toy building elements have also been disclosed in a number of U.S. patents. For example, in U.S. Pat. No. 1,371,619 (Greenstreet), wooden toy building blocks are disclosed which include a square block having notches in the center of each side. In U.S. Pat. No. 1,163,851 (Pringle), a construction set including square and rectangular members with slots along the sides for interconnection is disclosed. The slots have a length so that a connecting board extends from the mid point of the rectangular member when two such members are perpendicularly connected with mating slots received in one another.
In U.S. Pat. No. 3,855,748 (Thomas), a playground assembly set including large and small rectangular members is disclosed. The large elements include a pair of slots along each edge while the smaller members have diagonal slots located at each corner. Another set of toy building elements is disclosed in U.S. Pat. No. 2,278,327 (Magnus et al). In this patent, a number of blocks which are rectangularly shaped and have slots along oposite edges are disclosed. The blocks interconnect with one another and are commonly made of wood although plastic, plywood, cardboard, or other suitable materials can be used. Similar shaped blocks are also disclosed in Swiss Pat. No. 436,079 (Kolliker-Koppel). Snap acting disc shaped elements are also disclosed in U.S. Pat. Nos. 2,984,935 (Beck) and 3,177,611 (Beck).